Influenza (Adult and Pediatric)

Why focus on influenza?

Influenza epidemics occur almost every winter. The impact varies from year to year but it is estimated that each year up to 20% of the US population becomes ill with influenza. Attack rates are even higher in children. Rates of hospitalization for children younger than 2 with influenza equal the rate in the elderly. Annually, up to 400,000 people are hospitalized and up to 40,000 people die from influenza and related complications.

What is the definition of influenza?

Influenza is an acute respiratory illness caused by influenza A, B and C viruses. Of these, influenza A and B are thought to cause significant disease. Infections due to influenza B usually cause a milder infection. Influenza A viruses are further categorized into subtypes by the 2 major surface protein antigens: hemagglutinin (H) and neuraminidase (N). In the northern hemisphere, annual epidemics of influenza typically occur during the fall or winter months. However, the peak of influenza activity can occur as late as April or May, and the timing and duration of flu seasons remain variable.

The Centers for Disease Control and Prevention define a case of influenza-like illness as fever AND either cough or sore throat.

Who should receive antiviral prophylaxis for influenza?

Indiscriminate use of chemoprophylaxis promotes resistance to antiviral medications and reduces antiviral medication availability for treatment of persons who are severely ill. Prophylaxis with oseltamivir may be considered in the following situations:

Very high-risk, asymptomatic patients if ≤48 hours have elapsed since exposure to a person with known or suspected influenza infection:

Highly immunosuppressed patients including:

Hematopoietic stem cell or organ transplant recipients

Patients currently receiving chemotherapy or radiation

HIV or other immunodeficiency with CD4 percentage <15%

Duration of prophylaxis: 7 days

Profoundly immunocompromised patients during periods of high influenza activity (with or without known exposure to persons with influenza infection):

Infants <3 months: antiviral prophylaxis is not recommended due to limited data in this age group

Breakthrough infections

Patients receiving prophylaxis should be instructed to seek medical evaluation if they develop a febrile respiratory illness that might indicate influenza. Infectious Diseases consultation is recommended for patients who develop influenza infection while taking or shortly after taking antiviral chemoprophylaxis.

At other times between October and May, when influenza is not widespread (See CDC weekly US map) test the following patients with influenza-like illness (fever and either cough or sore throat):

Nasopharyngeal (NP) swab or aspirate are the optimal samples for detection of Influenza. These samples can be difficult to obtain in adults and a nasal or throat swab are acceptable alternatives, although they may have slightly lower sensitivity for virus detection. NP swabs should be collected using a flexible, rayon, mini-tipped swab. Throat and nasal swabs should be collected using a regular culturette.

Preferred test for persons ≥2 years of age (PCR for influenza A/B)

Preferred test for persons <2 years of age (PCR for influenza A/B and RSV, can be done on a single swab)

PCR for influenza A/B has a reported sensitivity of 100% and a specificity of 98.6% for influenza A and 100% sensitivity and specificity for influenza B, respectively. Tracheal aspirates are not acceptable for testing due to the viscous nature of these specimens.

If PCR is not readily available, rapid antigen tests for influenza A and B may be used. However, the sensitivity of the rapid antigen tests is low so false-negative results are very common.

Culture and serologic testing have no role in the primary diagnosis of acute disease.

Testing is most likely to detect influenza in the first 1-3 days after symptom onset, when the the highest levels of virus are shed but PCR can be positive for as long as 7 days following onset. When influenza is widespread, treatment decisions should be based on clinical presentation rather than test results.

Indications for hospitalization

Adults:

Hypotension

Dehydration

Renal failure

Respiratory distress

Altered mental status

Social concerns (elderly, living alone)

Children:

Dehydration

Respiratory distress

Suspicion of bacterial pneumonia

Children younger than 12 months who are more than mildly ill-appearing

Use clinical judgment when determining if patient is unstable and in need of hospitalization.

Instruct the patient/caregiver to notify the staff on arrival at the clinic that patient has a febrile respiratory illness

Desk staff should be alerted that the patient should be masked with a surgical-style mask and further instructed on respiratory etiquette and hand hygiene. The patient should then be placed in an examining room as soon as possible or in a separate area of the waiting room away from other patients

When examining the patient or while taking a nasopharyngeal swab, put on gloves and a droplet mask (mask that covers mouth and nose and also provides eye protection)

High-risk patient?

For high-risk patients, test if possible but do not delay treatment for test results.

Factors that increase risk for significant disease and complications:

Age <12 months or >65 years

Patient <19 years on daily aspirin therapy

Resident of nursing home

Hospitalized patients

Pregnancy or lactation

Postpartum (within first 14 days)

Morbid obesity (body mass index ≥40)

Pulmonary disease (including asthma)

Cardiovascular disease (excluding hypertension)

Liver or renal disease

Neurologic or neurodevelopmental disorders

Airway abnormalities

Chronic metabolic disease (including diabetes)

Hemoglobinopathies

Immunodeficiency

HIV or other immunodeficiency with CD4 percentage <15% or <200 cells/mm3

Testing and treatment for influenza is not likely to provide significant benefit to previously healthy individuals even if within 48 hours from onset of symptoms. Provide symptomatic treatment and reevaluate if symptoms worsen.

Condition improved?

Return to daycare, school, and/or work

Patients with influenza should be excluded from daycare, school, and/or work until at least 24 hours after fever resolution (without the use of fever-reducing medicines).

Health care workers caring for hematopoietic stem cell transplant patients in protective environment: Exclude from work for 7 days from symptom onset or until 24 hours after the resolution of fever, whichever is longer. Alternatively consider temporary re-assignment to other areas.

Follow up with primary provider for immunization

No routine tests or referrals are needed for influenza follow up

Ensure that an influenza vaccine is administered during the current season, if not already administered, and annually (See CDC: Seasonal Influenza Vaccination Resources for Health Professionals). Influenza vaccine protects against multiple strains of influenza, therefore, vaccination is indicated even after an illness with influenza to protect against the other strains.